1. Field of the Invention
The invention relates to discharge lamp of the short arc type. The invention especially relates to discharge lamp of the short arc type which is used as the light source for a projection apparatus, and a discharge lamp of the short arc type which is used as the light source for semiconductor exposure and which is filled with mercury.
2. Description of Related Art
In a discharge lamp of the short arc type which is used for the above described purpose, conventionally, in the arc tube, there are large electrodes in order to suppress a temperature increase of the electrodes and to prevent thermal wearing of the electrodes. Furthermore, in the hermetically sealed tubes which border the arc tube, electrode rods which support the electrodes are each inserted into a glass cylindrical retaining body which is used to reduce the amount of contraction of the respective hermetically sealed tube, and is welded to the inside of the hermetically sealed tube in order to thus prevent damage to the hermetically sealed tube.
In this lamp, to improve the illumination property, one end of a trigger wire is wound onto one of the hermetically sealed tubes. The other end of this trigger wire is located along the outside surface of the arc tube on the other hermetically sealed tube.
There are cases in which the trigger wire is electrically connected to one of the electrodes. On the other hand, there are cases in which the trigger wire is not connected to any of the electrodes. In both cases, when a breakdown voltage is applied by an ignitor between the electrodes, it is possible to contribute to operation of the lamp.
A conventional technology in which the type of arrangement of these trigger wires is improved, a reduction of the breakdown voltage is attempted and thus a discharge lamp of the short arc type is reliably operated regardless of the type of operating device, is disclosed in Japanese patent disclosure document HEI 2-199766 (Patent document 1) and Japanese patent disclosure document HEI 2-210750 (Patent document 2)
In the field of semiconductor exposure, there has recently been a demand for a lamp with a still greater radiance of UV radiation for purposes of achieving an increase of the throughput in the exposure process. Furthermore, in the field of image display, there is a demand for a still larger lamp in order to increase the illumination of the screen. Here, there is a tendency for the lamp input power to become greater.
Therefore, in this lamp, the distance between the electrodes becomes large and the gas filling pressure is also high. The reason for increasing the distance between the electrodes is the following:
In the case of a small distance between the electrodes, the phenomenon occurs that the electrode tips begin to melt due to heat. To prevent this, it is necessary to increase the distance between the electrodes.
The reason for increasing the gas filling pressure in a lamp for semiconductor exposure is to increase the radiance of the UV radiation. For this purpose, a means for increasing the pressure of a buffer gas, such as argon, krypton, xenon or the like, is used. In a lamp for image display, a means for increasing the amount of xenon gas to be added is used to increase the light intensity, by which the gas filling pressure becomes high.
In these lamps, the electrodes acquire a large shape (especially with respect to the outside diameter of the body of the respective electrode), and the inside diameter of the hermetically sealed tube becomes large. Therefore, there is the tendency for the thickness of the cylindrical retaining body to increase in order to prevent the amount of contraction of the hermetically sealed tube from increasing.
However, if the lamp arrangement is changed in the above described manner, the lamp cannot be operated without increasing the breakdown voltage. But, if the breakdown voltage is increased, the amount of the surge which returns to the power source, i.e., the so-called noise, is increased. In this way, the disadvantage arises that the power source is destroyed or that the arrangement of the circuit for preventing destruction of the power source becomes complicated. This means that the disadvantage arises that the operating properties of the lamp are degraded.